There are numerous brush wear detection systems which have been proposed to detect the wear of brushes used in dynamoelectric machinery. As is known, detection of critical brush wear is of great advantage, both in allowing replacement of brushes in the dynamoelectric machine at a time when the machine would otherwise be inactive, and in preventing damage to the commutator or slip ring of such a machine by current lead wires embedded in the brush contacting the commutator or slip ring as the brush wears. To provide a brush wear signal, a sensing wire may be embedded in a brush and protrude into the brush a short distance further than the embedded current lead, in the manner disclosed by British Pat. No. 1,262,486 dated Feb. 2, 1972, or by coating a small wire about its periphery and end with a thermoplastic material, chemically abrading its surface, and glueing it into a hole provided in such a brush, or under a loop-type lead of such a brush, as disclosed in co-pending patent applications Ser. Nos. 118,943; 118,944 and 118,945 filed Feb. 6, 1980. Brush holders, or brushes and brush holders have also been modified to provide a critical wear signal, such as disclosed in U.S. Pat. Nos. 2,193,172; 2,691,114; 3,523,288; 3,609,429; 3,898,492; 4,024,525; 4,121,207 and 4,172,988.
Various means have been proposed to provide a brush wear indication based on such a brush wear signal, such as by using inverting transistors where necessary to control a silicon controlled rectifier, as in U.S. Pat. No. 4,024,525, or by commparing the voltage present upon a line providing the brush wear signal with an oscillating voltage, as disclosed in the above-mentioned co-pending patent applications. These circuits provide the desirable feature of providing a sustained warning following a momentary brush wear signal. However, since these circuits utilize semiconductor devices, they must be located away from temperature-producing components, and are also susceptible to leakage currents due to brush dust from worn brushes, yielding a false indication of critical brush wear. At initial assembly, connections to brush wear sensing wires may be insulated with an insulating varnish, but, when brushes are replaced later, in the field, either the time or materials necessary to perform such an insulating step may not be available. If a brush wear indication is based on a voltage on a brush wear sensing wire due to contact with the commutator or slip ring, or a voltage due to contact between a brush and a contact or switching element, a false brush wear signal may be provided by leakage current through brush dust, which usually simulates a high voltage such as would be caused by critical wear of a positive brush.
Latching relays and the like have been made, which are capable of performing a memory function for maintaining a critical brush wear indication until repairs can be conveniently made. Such a function can be performed with two relay or solenoid devices, with a pivoted armature or contact carrier maintained in position by over-center linkages, over-center springs, permanent magnets, residual coil core magnetism, and the like. Some such devices contain numerous moving parts, and are complex in design, difficult to manufacture, and unreliable in operation. Such devices relying on friction or magnetism are apt to give false indications when subjected to vibration. Previous known devices do not provide an electromechanical memory device such as a latching relay which is simple and rugged in construction, and resistant to the vibration levels such as may be found in the preferred usage for such a structure, mobile material handling vehicles.
In accordance with the instant invention, a simplified electromechanical memory device shown as a latching relay, and a critical brush wear detecting system utilizing such a memory device are disclosed. In basic structure, the memory device shown as a latching relay utilizes two conventional relays, mounted perpendicular to each other, portions of the armature of each being capable of substantially perpendicularly over-lapping a portion of the armature of the other relay. It should be noted that for the purposes of this application, the word relay includes solenoid-devices which are not provided with electrical contact pairs, as well as conventional relays, since it will be obvious that a functional device can be built in accordance with the invention without providing electrical contacts on both the solenoid-devices shown and identified as relays herein.
When one of the two perpendicularly-mounted relays is energized, and the second is de-energized, the armature of the second relay will overlap the armature of the first relay, maintaining it in energized position. If the second relay is subsequently energized, its armature will move to energized position, releasing the armature of the first relay. The first relay being de-energized at this time, its armature will maintain the second relay armature in energized position. Should both relays be energized at the same time, an unrealistic condition, whichever relay remains energized the longest will have its armature latched in energized position.
To use such a novel memory element in a critical brush wear detecting system according to the invention, one relay, providing a detection or trip or warning function hereinafter referred to as a set relay is provided with a pair of electrical contacts which actuate a critical brush wear warning device. The second relay, hereinafter referred to as a reset relay, may be energized to remove the critical brush wear indication after critically worn brushes have been renewed, or after the warning system according to the invention has been tested, and is provided with one or more pairs of contacts connected in series with a relay coil of the set relay, to disable this relay coil after it has been energized to provide a critical brush wear indication. In this manner, the relay coil can be used over a wide range of voltages, and made with such a low impedance as to be insensitive to leakage current caused by brush dust. The relay coil may be momentarily severely overloaded, but will not be heated and damaged, since it will be disabled as soon as its armature is moved to energized position, where it will be maintained by the armature of the reset relay.
In the illustrated embodiment of a brush wear warning system according to the invention, the first or set relay is provided with two coils, one for each of the two polarities found in a conventional dynamoelectric machine, one coil being connected to sensing wires associated with positive brushes, and the other coil being associated with negative or ground brushes. Although embedded sensing wires are disclosed, it would be obvious to substitute mechanical switching elements which provide a switch closure upon critical brush wear. However, such structures are believed undesirable as being unduly fragile, or as applying undesired pressures which may cause a brush to bind in its brush holder.
Therefore, it is a first object of the invention to provide a simplified structure for a latching relay. It is a feature of the invention that two conventional relays are mounted at right angles to each other, in a perpendicular relationship, so that the armature of each, in de-energized position, may overlap a portion of the opposite relay armature and maintain it in energized position. It is an advantage of the invention that a memory device or latching relay may be formed using the simple and rugged structure of two conventional relays.
It is a second objective of the invention to provide a brush wear warning system using the novel structure of a memory device or latching relay according to the invention. It is a feature of the invention that a first or set relay of a memory device according to the invention is connected to a means for providing a critical brush wear signal, through contacts controlled by a second or reset relay of the memory device, the first relay thus being electrically de-energized when its armature is mechanically latched in energized position. It is an advantage of the invention that a wide range of voltages provided by the means for providing a critical brush wear warning signal may be accommodated by a memory device according to the invention, since the first or set relay coil will be energized only momentarily, and can safely maintain even a severe overload for a short time.